Silicon nitride, sialon and similar ceramics characteristically not only have a smaller specific gravity and are more corrosive-resistant than steel but are also insulating. Accordingly, if ceramics are adopted as a material for a rolling contact member that is a component of rolling bearings (including hub units) each including a race member and a rolling element, for example, are adopted as a material for the race member and the rolling element, they can provide bearings and the like reduced in weight and also prevent rolling bearings from having short lives as their components corrode and are thus damaged or electrolytically corrode.
Furthermore, a hub unit, which is a type of rolling bearing, is often used in an environment having a possibility of receiving moisture therein and hence having insufficient lubricity. Ceramic rolling elements, race members and similar rolling contact components are characteristically less damageable in such an insufficiently lubricating environment as above. Accordingly, for example, a hub unit with a rolling contact component formed with ceramics adopted as a material can exhibit improved durability when it is employed in an insufficiently lubricating environment.
Meanwhile, in a wind turbine generator, rotation of a main shaft connected to a blade, which is rotated by force of wind, is speeded up by a step-up gear box and is transmitted to an output shaft. Accordingly, the output shaft is rotated, which causes rotation of a rotor of a power generator, thereby generating electric power. Here, to attain improved efficiency in power generation, a demand arises for weight reduction of rolling bearings for such a wind turbine generator. The rolling bearings for a wind turbine generator include: a main shaft bearing for supporting the main shaft of a windmill used for the wind turbine generator; a power generator bearing for rotatably supporting a rotation member, which rotates according to rotation of the main shaft, such as a rotor shaft connected to the rotor of the power generator, relative to a member disposed opposite to the rotor shaft; a step-up gear box bearing for rotatably supporting the rotating shaft, which rotates in an inner portion of the step-up gear box, relative to a member disposed opposite to the rotating shaft; and the like. Further, such rolling bearings for the wind turbine generator operate depending on increase/decrease of force of wind, and speed of rotation is therefore greatly increased/decreased. As a result, lubricity is likely to be insufficient. Further, the rolling bearings for a wind turbine generator are occasionally installed in locations in which it is difficult to do maintenance of the rolling bearings. Accordingly, a demand arises in maintenance-free rolling bearings. High durability is required in an environment of insufficient lubricity.
Further, in the power generator, current may flow in the inner portion of the power generator bearing due to its device structure. Furthermore, wind turbine generators, which are usually installed at a high altitude, may be struck by lightning, which may cause current to flow in the inner portion of a rolling bearing for a wind turbine generator. The current flowing in the inner portion of the rolling bearing for a wind turbine generator causes generation of spark between a race member such as a bearing ring and a rolling element such as a ball or a roller, both of which are components of the rolling bearing for a wind turbine generator. This may result in generation of electrolytic corrosion. The electrolytic corrosion causes damage on a raceway surface of the race member or the rolling element, resulting in decreased life of the rolling bearing for a wind turbine generator.
As described above, as compared with steel, ceramics such as silicon nitride and sialon characteristically have a smaller specific gravity, are insulative, and are less likely to be damaged in an environment of insufficient lubricity. Hence, by employing such ceramics as materials for rolling contact members constituting a rolling bearing for a wind turbine generator such as a race member or a rolling element in the rolling bearing for a wind turbine generator, the weight of the bearing can be reduced and the decreased life resulting from electrolytic corrosion can be prevented, thus providing the rolling bearing for a wind turbine generator with a rolling contact member highly durable in an environment of insufficient lubricity.
However, silicon nitride, sialon and similar ceramics require higher production cost than steel, and adopting ceramics as a material for components of rolling contact members of rolling bearings disadvantageously increases their production costs.
In recent years, there has been developed a method of producing β-sialon, a type of ceramics, inexpensively by a production process including combustion synthesis (see Japanese Patent Laying-open No. 2004-91272 (Patent Document 1), Japanese Patent Laying-open No. 2005-75652 (Patent Document 2) and Japanese Patent Laying-open No. 2005-194154 (Patent Document 3) for example). This allows one to consider adopting β-sialon as a material for components of rolling bearings to produce them inexpensively.    Patent Document 1: Japanese Patent Laying-open No. 2004-91272    Patent Document 2: Japanese Patent Laying-open No. 2005-75652    Patent Document 3: Japanese Patent Laying-open No. 2005-194154